eventfs

Eventfs is a specialized userspace filesystem where each directory serves as process-owned message queue. Its semantics differ from POSIX in the following ways:

• Every non-empty directory has a head and tail symlink.
  • The head symlink always points to the oldest file in the directory.
  • The tail symlink always points to the newest file in the directory.
• Unlinking head does not remove the head symlink, but instead unlinks the file pointed to by head. Similarly, unlinking tail unlinks the file that tail points to.
  • head is atomically retargeted to the next-oldest file.
  • tail is atomically retargeted to the next-newest file.
• By default, each directory shares fate with the process that created it. If the creator process dies, the directory and its contents cease to exist.
  • If the directory has the user.eventfs_sticky extended attribute set, the directory persists until explicitly removed.
• There are no nested directories.
• There is (currently) no rename(2).

Sample Use-Cases

The motivation behind eventfs is to provide an efficient but accessible userspace IPC system that is portable across multiple *nix. It implements reliable message multicasting while avoiding excessive copying, and offers access controls, quotas, channel-sharing, and readiness notification through familiar filesystem semantics.

It is currently used to:

• Give libudev-compat clients a way to receive device events while guaranteeing that they do not leave behind any residual state once they exit.
• Provide namespaceable event channels for sending messages between containers.

Example

Here is a sample execution trace, where a Python script creates a directory called demo and goes to sleep. Other processes create files in demo, and eventfs ensures that the directory's head and tail symlinks always point to the oldest and newest files. Once the demo directory is empty, the head and tail symlinks vanish. Once the Python script exits, the demo directory vanishes.

$ cat ./demo.py
#!/usr/bin/env python

import os
import time 
import sys

os.mkdir( sys.argv[1] )
time.sleep(10000)

$ mkdir events 
$ ./eventfs events
$ ls -l events
total 0
$ ./demo.py events/demo &
[1] 4014
$ ls -l events/demo
total 0
$ echo "message text" > events/demo/msg
$ ls -l events/demo 
total 0
lrwxrwxrwx 1 root root  3 Sep  4 12:11 head -> msg
-rw-r--r-- 1 jude jude 26 Sep  4 12:11 msg
lrwxrwxrwx 1 root root  3 Sep  4 12:11 tail -> msg
$ echo "message text 2" > events/demo/msg2
$ ls -l events/demo
total 0
lrwxrwxrwx 1 root root  3 Sep  4 12:11 head -> msg
-rw-r--r-- 1 jude jude 30 Sep  4 12:12 msg
-rw-r--r-- 1 jude jude 30 Sep  4 12:12 msg2
lrwxrwxrwx 1 root root  4 Sep  4 12:11 tail -> msg2
$ rm events/demo/head
$ ls -l events/demo 
total 0
lrwxrwxrwx 1 root root  4 Sep  4 12:14 head -> msg2
-rw-r--r-- 1 jude jude 30 Sep  4 12:12 msg2
lrwxrwxrwx 1 root root  4 Sep  4 12:11 tail -> msg2
$ rm events/demo/tail
$ ls -l events/demo
total 0
$ fg
./demo.py tmp/events
^CTraceback (most recent call last):
  File "./demo.py", line 8, in <module>
    time.sleep(10000)
KeyboardInterrupt
$ ls -l events
total 0
$ fusermount -u events

Dependencies

• fskit
• libpstat

Building

To build:

    $ make

Installing

To install:

    $ sudo make install

By default, eventfs will be installed to /usr/local/bin. You may set PREFIX to control the installation directory, and DESTDIR to set an alternate installation root.

Running

To run:

    $ ./eventfs [-c /path/to/config/file] /path/to/mountpoint

It takes FUSE arguments like -f for "foreground", etc. See fuse(8).